多点广播
Datagram只允许数据报发送给指定的目标地址，而MulticastSocket可以将数据报以广播方式发送到数量不等的多个客户端。
IP协议为多点广播提供了这批特殊的IP地址，这些IP地址的范围是224.0.0.0至239.255.255.255。
MulticastSocket 常用的方法：
MulticastSocket(int port) ：创建多播套接字并将其绑定到特定端口。创建一个MulticastSocket对象后，还需要将该MulticastSocket加入到指定的多点广播地址，如果结束也需要脱离多点广播地址。
void joinGroup(InetAddress mcastaddr) ：加入多播组。
void leaveGroup(InetAddress mcastaddr) ：离开多播组。
void setLoopbackMode(boolean disable) ：启用/禁用多播数据报的本地回送。true 表示禁用 LoopbackMode。
案例：群聊
1 import java.io.IOException;
2 import java.net.DatagramPacket;
3 import java.net.InetAddress;
4 import java.net.MulticastSocket;
5 import java.util.Scanner;
6
7 public class Chat {
8 private volatile static boolean exit = false;
9 private static Scanner input = new Scanner(System.in);
10 private static String username;
11
12 public static void main(String[] args) throws IOException {
13 MulticastSocket socket = new MulticastSocket(9999);
14 InetAddress ip = InetAddress.getByName("230.0.0.1");
15 socket.joinGroup(ip);
16 socket.setLoopbackMode(false);
17
18 System.out.print("请输入用户名：");
19 username = input.nextLine();
20
21 SendThread s = new SendThread(socket,ip);
22 ReceiveThread r = new ReceiveThread(socket);
23
24 s.start();
25 r.start();
26
27 try {
28 s.join();
29 } catch (InterruptedException e) {
30 e.printStackTrace();
31 }
32
33 socket.close();
34 input.close();
35 }
36 static class SendThread extends Thread{
37 private MulticastSocket socket;
38 private InetAddress ip;
39
40 public SendThread(MulticastSocket socket,InetAddress ip) {
41 super();
42 this.socket = socket;
43 this.ip = ip;
44 }
45 public void run(){
46 try {
47 while (!exit) {
48 System.out.print("输入广播消息：");
49 String message = input.nextLine();
50 if ("bye".equals(message)) {
51 exit = true;
52 break;
53 }
54
55 byte[] data = (username+":"+message).getBytes();
56 DatagramPacket dp = new DatagramPacket(data, data.length, ip, 9999);
57
58 socket.send(dp);
59 }
60 } catch (IOException e) {
61 e.printStackTrace();
62 }
63 }
64
65 }
66 static class ReceiveThread extends Thread{
67 private MulticastSocket socket;
68
69 public ReceiveThread(MulticastSocket socket) {
70 super();
71 this.socket = socket;
72 }
73
74 public void run(){
75 try {
76 while (!exit) {
77 byte[] data = new byte[1024];
78 DatagramPacket dp = new DatagramPacket(data, data.length);
79 socket.receive(dp);
80 String str = new String(data, 0, dp.getLength());
81 System.out.println(str);
82 }
83 } catch (IOException e) {
84 exit = false;
85 }
86 }
87
88 }
89 }